Change drag button from right to left mouse button for CAD panning

[Copilot]

This PR implements the feature request to change the drag button for panning CAD files from the right mouse button to the left mouse button, providing a more intuitive user experience.

Changes Made

• Mouse interaction: Modified OnCADPictBoxMouseDown in CADImaging.cs to detect MouseButtons.Left instead of MouseButtons.Right for initiating drag operations
• Documentation: Updated README.md usage instructions to reflect the new behavior

Behavior Changes

Before:

• Right press and drag to pan the view
• Left double-click to reset scaling
• Mouse wheel to zoom

After:

• Left press and drag to pan the view
• Left double-click to reset scaling (unchanged)
• Mouse wheel to zoom (unchanged)

The change maintains full compatibility with existing functionality. Double-click reset scaling continues to work seamlessly with the left mouse button, as Windows Forms handles double-click as a separate event from the drag operations.

Technical Details

This is a minimal, surgical change affecting only the mouse button detection logic. The drag operation itself, cursor management, and all other mouse interactions remain identical to ensure stability and consistency.

Fixes #2.

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